For over four decades, aminoacyl-tRNA synthetases (AARSs) were thought of as essential housekeeping proteins that catalyze the aminoacylation of tRNA molecules as part of the decoding of genetic information during the process of protein translation. AARSs have been extensively studied in this respect, and many of their full-length sequences were cloned for sequence analysis and to provide a rich source of biochemical experimentation. Some fragments of AARSs, and associated proteins, however, possess unexpected activities not associated with aminoacylation, including extracellular signaling activities that modulate pathways beyond protein translation. Generally, these unexpected activities are not observed in the context of the full-length or parental protein sequences; instead, they are observed following removal or resection of AARS protein fragments from their parental sequences, or by expressing and sufficiently purifying fragment AARS sequences and then testing for novel, non-synthetase related activities.
While the full-length sequences of AARS have been known for some time, no systematic experimental analysis has been conducted to elucidate such AARS protein fragments, or protein fragments from related or associated proteins, or to evaluate the potential role of the full length AARS proteins for novel biological activities outside of the context of amino acid synthesis. In portions of this specification, such AARS protein fragments, AARS domains, or AARS alternative splice variants are referred to herein as “resectins”. In its broadest context, the term “resectin” refers to a portion of a protein which has been excised or restricted (either by means of proteolysis, alternative splicing, mutagenesis, or recombinant genetic engineering) from the context of its native full-length or parental protein sequence, which often otherwise masks its novel biological activities. Likewise, no systematic experimental analysis has been conducted to explore the use of such resectins as biotherapeutic agents, diagnostic agents, or drug targets in the treatment of various medical conditions, or their potential association with human diseases. As essential housekeeping genes with a known function in mammals that is critical to life, AARSs were neither considered as drug targets in mammals, nor were they parsed out by standard genomic sequencing, bioinformatic, or similar efforts to identify resectins having non-synthetase activities. Standard biochemical research efforts have similarly been directed away from characterizing the biological properties of AARS resectins and their potential therapeutic and diagnostic relevance, mainly due to the previously understood role of their corresponding full-length parental AARSs.
In higher eukaryotic cells, AARSs are organized into a high molecular mass multi-synthetase complex (MSC), where nine AARSs and three accessory proteins are bound together (reviewed in Guo et. al.; (2010) FEBS Lett. 584(2): 434-442). The three non-tRNA synthetase accessory proteins are thought to serve as scaffold proteins providing structural integrity to the protein complex, these accessory proteins include p43 (also named AIMP1), p38 (also named AIMP2) and p18 (also named AIMP3). This protein complex may serve to promote ordered protein synthesis, but has been hypothesized to serve as a reservoir of regulation molecules for functions beyond the role of its' components in protein synthesis.
The accessory protein p38 has been shown to possess activities outside of the multi-synthetase complex and the realm of protein synthesis. Although genetic disruption of p38 (AIMP2) in mice causes neonatal lethality, studies using p38 (AIMP2)-deficient mice showed cell type specific defects in lung differentiation and respiratory distress syndrome that could not be attributed to directly to protein synthesis defects. These defects have been instead been attributed to a role for p38 in regulation of c-myc lung cell differentiation via FBP interaction and degradation. (Kim, M. J. et al. (2003) Nat. Genet. 34, 330-336). Other studies have identified p38 (AIMP2) 1) as a substrate for the parkin protein facilitating neurodegeneration, 2) as a promoter of apotosis via interaction with p53 to prevent its degradation. (Ko, H. S. et al. (2005) J. Neurosci. 25, 7968-7978; Han, J. M. et al. (2008) Proc. Natl. Acad. Sci. USA 105, 11206-11211).
Recently a splicing variant that results in p38 (AIMP2) lacking exon 2 (AIMP2-DX2) was discovered to be highly expressed in human lung cancer cells and patient tissues. Mice constitutively expressing AIMP2-DX2 showed increased susceptibility to carcinogen-induced lung tumorigenesis. Moreover, the expression ratio of AIMP2-DX2 to normal AIMP2 was increased according to lung cancer stage and showed a positive correlation with the survival of patients. (Choi J. W. et al. (2011) PLoS Genet. 2011 March; 7(3): e1001351).